1. Field of the Invention
This invention relates to a process for blow molding polyamide compositions. More particularly, this invention relates to a novel blow moldable composition which comprises a polyamide and an effective amount of one or more cyanate components.
2. Prior Art
While blow molded thermoplastics have been used for several years to manufacture automotive parts such as windshield washer fluid reservoirs, radiator overflow tanks, load floors, seat backs and shock absorber boots, the choice of resins has been limited mainly to high density polyethylene and polyester elastomers. Both have suitable processing ranges, but neither is noted for high-temperature resistance (both mechanical and chemical) or paintability. Nylon polymers, because of their inherent physical and chemical properties, can extend the range of product application for improved performance in high and low-temperature environments, and also can be painted.
However, the traditional array of nylon products has not included a true general-purpose blow molding resin. Although some have been used to a limited degree in the automotive industry and elsewhere, nylon resins in general have lacked broad applicability for blow molding because of the severely restricted processing latitude.
There are several properties which are necessary for blow molding resins. These resins should exhibit a high melt viscosity that is relatively independent of shear rate and a melt viscosity that has a relatively low dependence on processing temperature. Moreover, these resins should exhibit a large difference between processing temperature and freezing temperature (for crystalline resins) or a wide temperature range over which the resin will flow in the case of amorphous (non-crystalline) materials. Lastly, these resins should possess an optimum elastic compliance (melt elasticity) to provide the necessary melt strength for the parison to hang.
When the above criteria are applied to a typical nylon, the thermodynamic behavior of common nylon 66 homopolymer shows the resin lacking several of the characteristics important to blow molding. For example, in the case of nylon 66 homopolymer, the melting point is about 505.degree. F. and the freezing point is 450.degree. F. There are two significant opposing factors that result from this thermodynamic behavior. The relative proximity of freezing and melting temperatures dictates that the processing temperature be as high as possible above the freezing temperature. The greater this difference, the longer can be the parison drop time and hang time before portions of the parison cool to the freezing temperature. If any portion of the parison cools too close to the freezing temperature, that portion will be malformed or warped. On the other hand, increasing the processing temperature significantly above the melting temperature is self-defeating since nylon 66 is too fluid (lacks melt strength) at temperatures slightly above the melting temperature. The second factor is the temperature dependence of viscosity. This can be illustrated by an example.
If it is assumed that the surface cooling rate of a parison is about 2.degree. F./sec (radiative loss only) and the temperature dependence of melt viscosity is 130 poise/.degree. F., then the melt-viscosity change as the parison drops and just before blowing is at least 260 poise in the 2-sec time interval of the blow cycle. This viscosity change is so significant that the object will be thin in some areas where the viscosity is low and thick in others where the viscosity is high. But viscosity alone does not tell the complete story. Part of the problem is caused by the high absolute melt temperature of nylon 66, since radiative heat losses are a function of temperature to the fourth power.
Also, in those areas where the part is thicker, greater shrinkage will occur, which aggravates the problem of malformed articles. Both of these considerations make it very difficult to blow mold an unmodified nylon 66 homopolymer.
It could be argued that increasing the processing temperature to a value considerably above the freezing point would allow sufficient time before freezing and that would afford more uniform objects. However, in general, high melt viscosities are preferred for blow molding. Raising the temperature defeats this, and furthermore only increases the rate of radiative heat loss.
The situation is somewhat better with nylon 6 because of the greater disparity between melting and freezing points (437.degree. F. and 361.degree. F.) and the lower absolute melting and corresponding heat loss permit a longer time for parison drop. Additionally, the temperature dependence of the melt viscosity of nylon 6 is 40 poise/.degree. F. about one-third the value of nylon 66. Not surprisingly, most commercial nylon blow molding applications are based on nylon 6 or copolymers thereof, where the lower absolute melting and freezing points (and differences), lower freezing rates, and temperature-dependent viscosity coefficients are advantageous.
Additionally, a resin with lower crystallinity is preferred for blow molding because the freezing rate is retarded significantly. One way of doing this is through the use of copolymers, for example, a (50.50) 6/66 nylon copolymer with a corresponding crystallinity of 40%. Reducing the crystallinity even further requires higher orders of copolymers or modification by agents that retard the crystallinity.
Increasing the molecular weight of the nylon will obviously retard parison sag. However, this alone does not reduce the tendency of the parison to be malformed because it does not affect significantly the crystallinity and, hence, freezing and melting behavior.